The depolarization of the nerve membrane takes place through the influx of ____ ions.
A. Calcium
B. Potassium
C. Sodium
D. Magnesium

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Hint: Depolarization of nerve cells is important for nerve impulse transmission. The neuron membrane becomes depolarized when an influx of certain positively charged ions occurs.

Complete answer: The cells in the central nervous system are called neurons or simply nerve cells. These cells are excitable due to the electric gradient across its membrane. The cell body is like a typical cell, consisting of a nucleus, mitochondria, and other cell organelles. From the cell body numerous filaments extrude, these are called dendrites.
Axon is the elongated projection which extends out to form synaptic knobs. It is covered by a myelin sheath.
Normally these cells remain polarized, but when an appropriate stimulus is received, the neuron membrane permeability changes.
Due to the changed permeability influx of sodium (Na+) ions occur. It marks a decrease in negative electric charge inside the cell i.e. loss of polarity – hence it is called depolarization.

So, the correct option is C. Sodium

Note: Neurons receive the signal via the dendrites and dendritic branches. Using action potential it transmits signals along the axon to the synaptic terminals. When action potential begins, the sodium channels are opened letting Na+ ions move inside the cell.